National data show that 65 per cent of the UK's men are overweight or obese but the survey of over 2,000 men found only 40 per cent thought they were in these groups.
The study by leading cancer charity Cancer Research UK found that two thirds of those men questioned were unaware that being overweight or obese increases the risk of cancer, while 75 per cent did not know that being physically inactive also increases their risk.
The survey, released to launch Cancer Research UK's Man Alive Campaign, shows that men are not heeding health warnings to maintain a healthy body weight.
Only 22 per cent of men questioned eat five portions of fruit and vegetables per day and two thirds fail to meet the recommended target of 30 minutes of moderate exercise five times a week.
It is estimated that obesity may now be responsible for around 4000 cases of cancer in UK men every year. A recent review suggests that obesity increases the risk of bowel cancer by 60 per cent. Being obese also doubles the risk of being diagnosed with kidney, oesophageal and stomach cancer as well as being a risk factor for bladder cancer.
The charity called on men to lead a more active lifestyle and to "eat a balanced diet that is rich in fruit and veg and low in sugar and fat" to reduce their cancer risk.
In the last ten years the rate of obesity for British men has increased two thirds. After smoking, obesity is one of the biggest preventable causes of cancer.
Professor Jane Wardle, director of Cancer Research UK's Health Behaviour Unit said that although many British women are also overweight, "more of them know they have a problem and that's the first step in putting it right".
The Man Alive campaign is designed to raise awareness of cancer among men and highlight ways to reduce the risk.
"With rates of obesity for UK men growing faster than anywhere else in Europe, we are sounding a wake up call to all men and their partners to become more active, eat healthier diets and make sensible lifestyle choices," said Dr Lesley Walker, director of information at Cancer Research UK.